Basics of rooftop Solar PV

Solar PV panels (also known as solar PV modules) work by converting sunlight into electricity. They do not use the heat from the sun, and in fact can see a reduction in power output in hot climates (this is discussed in greater detail here)

The electricity generated by the PV panels is Direct Current (DC). This needs to be converted into Alternating Current (AC) using an inverter

The panels are mounted on the rooftop using special mounting structures

If solar power is required when there isn’t enough sunlight for the panels to generate electricity (such as at night), a battery backup is required

A charge controller is required to regulate the charging of batteries

These are the primary components of a rooftop solar PV plant. Other components include the cables, switchgear, fuses, etc.

As the amount of sunlight falling on the panels varies during the day (due to clouds, etc.), the power output from the panels also varies. As this variation in power could damage equipment, the inverter continuously matches the PV plant’s output to another source of steady power. Therefore a rooftop solar PV that generates AC power will always needs another source of power (whether the grid or diesel generator or batteries) to provide a reference voltage in order to function. If such a source of power is absent, the plant will not generate power even if there is ample sunlight.

Components of a rooftop solar PV plant

From the above, we can see that a rooftop solar PV plant primarily requires 3, and in some cases 5, components

PV modules (panels)

Inverters

Mounting structures

If battery backup is required

Batteries

Charge controller

PV modules (panels)

There are two kinds of modules: Thin-film, and Crystalline. Rooftop solar plants predominantly use crystalline panels because they are more efficient and therefore better suited to installations like rooftops where space is a constraint.

Panel efficiency

It should be noted that the efficiency of a solar panel is calculated with reference to the area it occupies. Two 250 Wp panels of different efficiency rating will generate the same amount of power, but occupy different amounts of space on your rooftop. A more detailed discussion on panelefficiency and its impact on space occupied by your rooftop plant can be found here.

Capacity rating

The capacity of a solar panel is denoted in terms of watts as Wp (watt peak). E.g., 250 Wp. This is the power output of the plant at 25°C. The capacity of the plant reduces at temperatures above 25°C and increases at temperatures below 25°C (more details here).

Inverters

Inverters are a very important component of your rooftop solar PV plant because they determine the quality of AC power you get, and also the kind of loads that can be powered with solar – different inverters support different levels of starting current requirements which affects the kind of machinery that can run on solar power. Inverters are also the only major component of your solar plant that are replaced during the lifetime of the plant.

Will I get power during a power failure?

Not all rooftop solar PV plants generate power during power failures. As previously mentioned, the solar inverter uses another source of power as a reference voltage. If the inverter is designed to use only grid power as a reference voltage, then the inverter will not be able to function in the absence of grid power and the solar plant will not generate power.

Therefore, if you are interested in rooftop solar to provide power during grid failures it is critical to choose an inverter that can use other sources of power as a reference voltage and continue to fuction even when the grid is down.

Kinds of inverters

Based on the explanation above, we can classify inverters into 4 types

Grid-tied –These inverters are primarily designed to supply the generated power to the grid and also power the load while grid power is available. This inverter will NOT generate power during a power failure, not only because it needs grid power as a reference voltage, but also because the inverter shuts down the system to stop sending power into the grid and avoids the risk of electrocuting utility personnel who are working to repair the grid (known as Anti Islanding)

Off-grid – These inverters do not work with the grid and are designed to work only with a battery backup or diesel generator in off-grid applications. They are suitable for applications where grid power is not available at all, but are not the right choice if you need your solar plant to work in conjunction with grid supply

Grid-interactive –These inverters work both with the grid supply and with either a battery backup or diesel generator to support the load even during a power failure.

Hybrid inverters (also known as Bidirectional or magical inverters) are a one system solution for a complete solar PV system. They can automatically manage between 2 or more different sources of power (grid, diesel, solar). They have inbuilt charge controllers, MPPT controller, Anti Islanding solutions, DC and AC disconnects and other features like automatic turning on/off of the diesel generator, automatic data logging, and various kinds of protection for the different components of the system, making them ideally suited for applications that require management of power from different sources

Mounting structures

Solar panels are mounted on iron fixtures so that they can withstand wind and weight of panels. The panels are mounted to face south in the Northern Hemisphere and north in the Southern Hemisphere for maximum power tracking. The tilt of the panels is at an angle equal to the latitude of that location.

The proper design of mounting structures is important to power plant performance as the power output from the PV plant will not be maximised if the mountings buckle and the panels are not optimally oriented towards the sun. In addition, improperly mounted panels present a ragged appearance that is not pleasing to the eye. Allowing sufficient air circulation to cool the PV panels is also an important factor that mounting structures should be designed for because, as mentioned above, rooftop PV plant output falls as temperatures rise above 25°C.

Trackers

Tracking is a way of mounting the panels through a mechanism that allows the panels to follow the sun as it moves across the sky. Single-axis trackers follow the sun as it moves from East to West during the day, while dual-axis trackers also follow the sun on its North-South journey over the course of a year.

Trackers can increase the power output from the PV plant but add significantly to both the initial cost of the plant and maintenance expenditure; utilisation of trackers should be decided on a case-to-case basis after performing a cost-benefit analysis over the lifetime of the rooftop plant.

Batteries

Reasons to use batteries

Make power available when the sun isn’t shining – This can be particular useful for applications where electrical consumption is greater during the night than in the day, such as BPOs that work on night shifts, or even residential apartments where most people are away during the day and at home during the night

Smoothen power delivery during the day – Clouds moving across the sun can suddenly reduce the output from your rooftop plant. A battery backup can ensure that the load gets sufficient power during such dips in plant output

Immediately cut-in during power failures – If space isn’t available for a large rooftop plant, solar panels with batteries can be used to support the load until a diesel generator can be turned on

Optimise time-of-use billing – If the utility charges different tariffs based on time of day, power from the batteries can be used to reduce consumption at those times when utility power is very expensive

Drawbacks to using batteries

Charge/discharge efficiency – Batteries and their charging equipment are not 100% efficient. There is a loss of energy both while charging and discharging the battery. Different models of batteries can have different charge/discharge efficiencies. If we lose 15% of the energy while charging and another 15% while discharging, we get back only about 72% of the power that was sent to the battery

Maintenance – Battery packs require careful maintenance. Maintenance isn’t limited to the physical condition of the battery (amount of electrolyte, cleaning of terminals) but also extends to the way we charge and discharge the battery. Repeatedly deep discharging the batteries, discharging before the battery has reached full charge, etc., are ways in which the life of the battery can be significantly reduced. Batteries can last as long as 10 years or give trouble within a few days, depending on how they are used

A battery pack can add about 25-30% to the initial system cost of a rooftop PV solar system for one day autonomy (storing an entire day’s output).

Due to the above drawbacks, we do not recommend coupling solar PV plants with battery backup unless absolutely necessary. If batteries are required, we urge you to perform a lifetime cost-benefit analysis to understand the effect on cost of solar power from your rooftop.

Charge Controllers

A charge controller regulates the DC power output from the rooftop solar panels that is used to charge the batteries. It provides optimum charging current, and protects the batteries from overcharging. There are two kinds of charge controllers

Pulse Width Modulated (PWM)

Maximum Power Point Tracking (MPPT)

MPPT charge controllers are more expensive than PWM but they offer much better performance in terms of efficiency, flexibility in solar panel plant configuration, and capacity supported.

Charge controllers that are integrated into the inverter are preferred as the inverter directs either grid power or solar power, based on availability and demand, to charge the batteries. This extends the battery life compared with using stand-alone charge controllers that allow parallel charging between grid and solar power at different power levels, damaging the battery

Maintenance of rooftop solar PV systems

The basic rooftop solar PV system has no moving parts and therefore requires very little maintenance. Additional components, such as trackers and batteries, can significantly increase the maintenance effort and expenditure.

Solar panels – These typically require little to no maintenance beyond having the dust cleaned off them. Solar panels can be expected to last for 25 years

Inverter – This can be affected by grid power quality or other issues common to power equipment such as humidity or short-circuits caused by insects, and may require some maintenance such as replacement of capacitors. The lifespan of an inverter is 5-10 years

Mounting structures – These typically last the lifetime of the plant and do not require maintenance, unless tracking systems are used

Tracking mechanisms involve moving parts that can wear out and/or break. The require lubrication, parts replacement, and sufficient room on the rooftop for maintenance access

Other parts of the system – Cabling, switchgear, fuses, etc. will require minor maintenance to ensure correct operation

Warranties

Other systems – Inverters, mounting structures, cables, junction boxes, etc. typically come with a 1 year manufacturer warranty which can be extended to 5 years

A detailed discussion on warranties and certification is provided here.

How do I choose a good vendor for a rooftop PV system?

Choosing a good vendor is critical to getting the most out of your rooftop PV system as carelessness in design or construction/installation can either significantly reduce the power output from your plant or deliver a plant that isn’t suited to your needs. A few things to keep in mind when selecting a vendor are

Supplier Background & Credibility

Ask for details of projects that they have already implemented

Check if they are MNRE authorised, or registered under your state’s energy development agency (or equivalent body)

Check if the supplied products have been manufactured in a ISO-9001 certified plant

Verify supplier’s claims about the product/component with datasheets available on the manufacturer’s website (e.g., if the supplier claims that the panels are suitable for coastal areas, check the product datasheet to see if it has cleared the salt mist corrosion test)

The cheapest vendor is not necessarily the best vendor. A vendor who has a well-established after-sales service network may quote a higher price but will provide greater benefits in the long run

When evaluating different vendors, ensure that the plant specification, and not just the description, is the same. E.g., 1 kW panel + 5 kW inverter may be sold as a 5 kW plant but is actually only a 1 kW plant. Similarly, 5 kW panels + 1 kW inverter is also a 1 kW plant. Such plants can be offered at a much lower price than a genuine 5 kW plant, but will not generate anywhere near the same amount of power

Price

The cheapest vendor is not necessarily the best vendor. A vendor who has a well-established after-sales service network may quote a higher price but will provide greater benefits in the long run

When evaluating different vendors, ensure that the plant specification, and not just the description, is the same. E.g., 1 kW panel + 5 kW inverter may be sold as a 5 kW plant but is actually only a 1 kW plant. Similarly, 5 kW panels + 1 kW inverter is also a 1 kW plant. Such plants can be offered at a much lower price than a genuine 5 kW plant, but will not generate anywhere near the same amount of power

How long does it take to install a rooftop PV system?

This can vary based on plant size, site conditions, and permissions required, but typically rooftop plants can be installed within two weeks to 3 months of the project being confirmed. It should be noted here that processing of subsidies may take much longer.

Takeaways

Not all PV plants generate power during power cuts; only grid-interactive plants do

Rooftop solar PV plants primarily comprise of

PV Modules (panels)

Inverters

Mounting Structures

Optionally

Batteries

Charge Controllers

PV plants have no moving parts and require very little maintenance, unless batteries or trackers are used

Warranties

Panels are typically warranted against manufacturing defects for 5 years, 90% of rated power output for 10 years and for 80% of rated power output up to 25 years

Other system components come with 1 year warranty extendable to 5 years

Vendors should be selected based on track record and ability to perform after-sales service

A rooftop PV plant may take a couple of weeks to 3 months to be installed, excluding time to process subsidies

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